A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen or air is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the electrolyte and a catalyst, producing anions and consuming the electrons circulated through the electrical circuit. The cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the first and second electrodes respectively are:H2→2H++2e−  (1)½O2+2H++2e−→H2O   (2)
An external electrical circuit withdraws electrical current and thus receives electrical power from the fuel cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions shown in equations 1 and 2. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, either stacked one on top of the other or placed side by side. The series of fuel cells, referred to as a fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds in the housing to the electrodes. The fuel cell is cooled by either the reactants or a cooling medium. The fuel cell stack also comprises current collectors, cell-to-cell seals and insulation while the required piping and instrumentation are provided external to the fuel cell stack. The fuel cell stack, housing and associated hardware constitute a fuel cell module.
Various parameters have to be monitored to ensure proper fuel cell stack operation and to prevent damage of any of the fuel cells. One of these parameters is the voltage across each fuel cell in the fuel cell stack hereinafter referred to as cell voltage. Ideally, differential voltage measurement is done at the two terminals (i.e. anode and cathode) of each fuel cell in the fuel cell stack. However, since fuel cells are connected in series, and typically in large number, measuring cell voltage for each cell is often prohibitively expensive and troublesome. A common compromise that is made in the art is measuring voltages across groups of cells within a fuel cell stack.
An example of this type of fuel cell voltage monitoring system is disclosed by Blair et al. in U.S. Pat. No. 5,170,124. In this patent, fuel cells within a fuel cell stack are divided into a plurality of groups and the voltage across each fuel cell group is measured. Then the measured voltage of each fuel cell group is normalized, i.e. averaged according to the number of fuel cells in the group. The normalized voltage of each fuel cell group is then compared with a reference voltage equal to a predetermined minimum voltage. If the normalized measured voltage is less than the reference voltage, an alarm can be activated. Another example of a fuel cell voltage monitoring system that utilizes averaged cell voltages is disclosed by Zeilinger et al in U.S. Pat. No. 6,432,569.
Although such fuel cell voltage monitoring systems alleviate the problems of measuring every cell voltage while meeting the requirement of monitoring cell performance, only average cell voltages are obtainable from these systems. In reality, it is more than likely that one or more cells in a fuel cell group has a voltage considerably lower than those of the others while the average cell voltage of that fuel cell group is still well above the predetermined minimum cell voltage. In this case, the fuel cell voltage monitoring system will not be able to detect the poor performance of the “bad cell” and activate an alarm and hence a corresponding recovery operation cannot be initiated in a timely manner. This will eventually lead to damage of the fuel cell stack and power shutdown.